This application relates to data communication equipment such as modems. More particularly, this invention relates to methods and apparatus for improving the performance of data communication equipment independent of the transmission technology being utilized.
In transmission of data over a channel, the system performance is aggravated by different impairments. Some impairments, such as intersymbol interference (ISI), phase jitter, etc., can be removed by properly utilizing particular procedures and processing as is well known in the art. Other impairments which can effect performance in different manners and degrees cannot be removed. These other impairments include, inter alia, Gaussian noise, impulse noise, and intermodulation distortion (IMD).
In light of the impairments of the channel, it is desirable to increase the signal/noise ratio of the system. The increase of signal/noise ratio is all the more important in high speed modems such as 19.2 kbit/sec modems. Among the more technically advanced techniques for increasing signal/noise ratio, some of which are presently being proposed as standards for a two-wire full-duplex modem (V.fast), are: Trellis shaping (see, e.g., G. David Forney, Jr., "Trellis Shaping", IEEE Information Theory Workshop; CCITT Study Group XVII & Working Parties, Geneva 15-23 Oct. 1990 Temporary Document 211-E); Trellis precoding (see, e.g., G. David Forney, Jr., "Trellis Precoding: Combined Coding, Precoding and Shaping for Intersymbol Interference Channels", IEEE Information Theory Workshop; CCITT Study Group XVII & Working Parties, Geneva 15-23 Oct. 1990--Temporary Document 212-E); fifty percent linear equalization (see, e.g., CCITT, Question 3XVII, WP XVII/1 "Preliminary Test Results--AT&T's Candidate Proposal for V.fast" and "DFE versus Preemphasis for V.fast" COM-XVII- E, April 1991); and shaping without Trellis (see, e.g., U.S. Ser. No. 07/651,563 to Cole and assigned to the assignee hereof). In fact, it is possible to utilize different combinations of the proposed techniques. Regardless of the techniques proposed, it is common practice to cause the signal being transmitted and carried by the channel to be as large as possible (i.e., typically 0.+-.0.5 dBm), thereby guaranteeing that the signal/noise ratio is as large as possible.
The function of the Trellis shaping, Trellis precoding, and shaping without Trellis techniques is to effectively maximize the distance (in n-space) between transmitted points of a constellation under average power constraints. By effectively increasing the distance between the points, the signal/noise ratio is increased. The function of the fifty percent equalization technique is to decrease noise enhancement of the linear equalizer, thereby improving the signal/noise ratio. As is disclosed in the above-referenced articles "Preliminary Test Results--AT&T's Candidate Proposal for V.fast" and "DFE versus Preemphasis for V.fast" which suggest utilizing fifty percent linear equalization, not only is the signal/noise ratio improved, but the impact of intermodulation distortion is decreased. As a result, the total error rate is reduced. The reason for the decrease in impact of intermodulation distortion, however, was not understood.
While each of the proposed techniques for V.fast has its own advantages and disadvantages, clearly it would be advantageous to increase modem performance above and beyond the proposed techniques.